Zero phase filtering can be performed by applying a digital recursive filter to an input signal in both forward and reverse directions, and combining the results to produce an output signal. The square root of the output signal is equal to the magnitude, and the phase of the output signal cancels to zero. Thus, the output signal has precisely zero lag. However, this bidirectional filtering technique does not produce a zero phase response in real time because it lacks future signal behavior.
A zero phase response in real time would be desirable.